It is not that common to find a bookstore in Seoul which sells only English books. All the biggies like Kyobo, Bandi and Lunis, YP Books deal mostly in Korean books and they have a very small section on foreign books where you can find some English Books. Mostly these shops would stock only bestsellers. So if you are looking for a specific writer or a book then finding such books would be a bit challenging in Korea.

There are a few good bookstores in Seoul which deals only in English books:

The first one is What the Book at Itaewon. It is next to next to foreign mart in Itaewon and they have a good collection of new as well as used books. They also have a website: what the book.

Then, there is The Foreign Bookstore at Noksapyeong. They have a huge collection of used magazines and books in almost all genres. To find this place you need to get down at Noksapyeong station (next to Itaewon) exit 2 and walk straight till you can spot a big board titled BOOKS on your right, across the road.

Other option would be to visit Itaewon Global Village Center. They have maintained a library for books received as donations from other expats. You need to make a deposit of 500o won for borrowing any of the books available their. For more details you can visit their blog here.

theyeogiyo.com also hosts a Book Exchange event regularly at Bundang. You can visit their website for more details.

P.S. the bookstores above do not have any technical books. All the technical books in the field of engineering or science are in Korean. So you can order them online via Amazon or what the book.